The present invention relates to an image projecting device which emits a scanning light beam into an eye of an observer to form an image on retina.
Conventionally, a retinal scanning display device which directly projects image on a retina by scanning weak light beams has been developed. The assignee of the present invention has suggested such a display device in Japanese Patent Publication No. 2874208. Such a retinal scanning display is typically known as a head-mounting display, which is configured such that an observer wears the display device, like spectacles, on the head. The retinal scanning display device is implemented with a wavefront curvature modulator which dynamically varies the wavefront curvature of the beam in order to provide a depth of the image formed on the retina of the observer.
Light emitted by a light source propagates as a light wave in all directions at the same phase, i.e., as isophase spherical wave. Depending on a distance between the light source and an observer, the radius of curvature of the spherical wave at the observer is different. That is, if the light source is close to the observer, an image of the light source is projected on the retina of the observer as an image having a small radius of curvature, while if the light source is remote, the image of the light source is projected on the retina of the observer as an image having a relatively large radius of curvature of the wavefront. The observer recognizes the difference of the radius of curvature and recognizes a natural perspective, or three-dimensional feelings.
In the conventional wavefront curvature modulator for a retinal scanning display, an optical system thereof is provided with a piezoelectric plate formed with a reflection surface thereon. A control voltage is applied to the piezoelectric plate so that the piezoelectric plate, and therefore the reflection surface is deformed. The light beam emitted by the light source is directed to the reflection surface, and the reflected beam is used for the retinal scanning. In this conventional wavefront curvature modulator, due to the deformation of the reflection surface (i.e., the piezoelectric plate), the wavefront curvature of the reflected beam is different from that of the incident beam. By varying the control voltage, the degree of change of the wavefront curvature of the reflected beam can be controlled. Recently, there is a requirement for an improved wavefront curvature modulating device which is capable of modulating the wavefront curvature at a higher frequency than the conventional device.